It's well-known that "deleting" a file from Windows doesn't fully remove it, but you may not understand just how much data can remain on your disk after a file is gone. Even fragments of a file such as a crucial e-mail or spreadsheet can contain valuable data. Browsing histories, cookies, and more can linger a long time. SecureClean ($US50, 14-day free trial) will remove all such detritus permanently.
The first thing SecureClean must do is scan your disk. If you do a deep scan, which is recommended for the first use, it can take a long time on even a moderately sized disk, so set it up when you will be away from your system for a few hours. When it's done, SecureClean gives you a detailed list of what it's found. You can ability to drill in to inspect specific items, such as cookies, or even the fragments of files left on your disk after deletion. Doing this can be instructive, as it can surprise you what remains, and even the items that are not human-readable can often be deciphered by utilities that know what to look for.
SecureClean 4 has a few issues. It hung on several files on my Windows 7/64 bit machine; this was a confirmed bug that will be fixed in an upcoming release. It specifically looks for IE and AOL cookies and temp files, but does not mention newer browsers like Firefox. Since items like browser history and cookies are often targets of choice if someone is looking for data, this can be a serious issue.
Once the scan is done, comes the cleaning. There are a good number of options for cleaning, allowing you to choose to not erase certain things (for example, wiping all your cookies can be annoying and may not be a security issue for you), or to add in some other options, such as erasing file names on NTFS drives, which are not stored in the same area as file content. SecureClean warns that doing so can be risky, due to how the names are stored, and it's up to you if a file name alone is something you want to scrub. SecureClean will also clean out "slack" in file sectors -- because a sector may be larger than the data contained in it (you could have the last 1K of a file in a sector, leaving 31K empty), even a sector which has been assigned to a new file can contain old data. SecureClean lets you choose to wipe this slack. How long the entire cleaning process takes is a function of how deep you wish to clean and how much there is to wipe.
SecureClean also includes a "file zapper", which is a secure file deletion tool. Drag a file into the zapper, and it will be fully erased, all of its data overwritten.
SecureClean has a number of nice features, but it's pushing the high end of pricing when similar functionality can be found in many utility programs. It is also aging, and I tend to be nervous when a program I trust to make low-level modifications to my hard disk might not be up-to-date on file storage mechanisms. I do, however, have to give it credit for finding some "deleted" data which I was truly surprised to see. I also like the level of control it gives you, a nice intermediate between "Erase All Or Quit" and tab after tab of subtle options which most users don't care about. The 14-day trial is a bit short, but it should be long enough to see if you like it.